In the VFFS apparatus, in the first step, a flat web of synthetic thermoplastic film is unwound from a roll and formed into a continuous tube in a tube-forming section. In the next step, the longitudinal edges of the film are sealed together to form a so-called “lap seal” or “fin seal.” The tube is pulled vertically downwards to a filling station and collapsed across its transverse cross-section at a sealing device below the filling station. The transverse heat seal at the collapsed portion of the tube makes an air-tight seal.
Next, the material being packaged enters the tube above the transverse heat seal in a continuous or intermittent manner and fills the tube upwards from the transverse heat seal. The tube is then allowed to drop a predetermined distance usually under the influence of the weight of the material in the tube. The jaws of the sealing device close again and collapse the tube at a second transverse section. Depending upon the nature of the material being packaged and the mode of operation of the process the sealing device transversely seals and severs the tube at the second transverse cross-section which may be at, above, or below the air/material interface in the tube.
The material-filled portion of the tube is now in the form of a pillow shaped pouch. The sealing device, thus, has sealed the top of the filled pouch, sealed the bottom of the next-to-be-formed pouch and separated the filled pouch from the next-to-be-formed pouch, all in one operation.
One VFFS apparatus of the type described above is a Prepac® IS-7E liquid packaging apparatus. A commonly used sealing device, the so-called impulse sealer, has electrically insulated sealing element mounted in the sealing jaws. In operation, the sealing jaws close and simultaneously an electrical current flows through the sealing element, for example, a wire. The jaws remain closed while the seal forms, but not while it cools and solidifies. Therefore, once the sealing jaws open, the synthetic thermoplastic film must provide a molten transverse seal that supports the weight of the flowable material, for example, liquid, in the next-to-be-formed pouch.
For reasons of economy, customers are demanding thinner and thinner films for the pouching of fluids. This can lead to one of two problems in commercially available film formulations: (1) inadequate sealability and toughness, or (2) insufficient stiffness.
Pouches made from commercially available films tend to suffer from defective seals, that is, a tendency to have weak transverse end and/or longitudinal seals even though the operating conditions of the impulse sealer have been optimized. Defective seals may lead to the phenomenon known as “leakers,” in which the flowable material, for example, milk, may escape from the pouch through pinholes that develop at, or close, to the seal. It has been estimated that leakers account for about 1-2% of the 1.3 liter milk pouch production.
As the pouch film is down-gauged for reasons of economy, its stiffness may also become an issue. Lack of stiffness may adversely affect the runnability of the film on a form, fill and seal apparatus and give poor stand-up properties for pouches in, for instance, a milk pitcher. However, higher stiffness has traditionally required a thicker gauge of the pouch film. But thicker gauge requires more material. The present invention, by localizing the stiffening material towards the outside of the multi-layer film, addresses both these issues in that the stiffness of the multi-layer film is not adversely affected even when its gauge thickness is reduced.